The invention is based on a priority application EP 02 360 060.4 which is hereby incorporated by reference.
The invention refers to a safety procedure for optical fiber systems transporting optical signals between a transmitter terminal and a receiver terminal, with at least one pump source to enhance said optical signal, by which safety procedure the optical input power fed into said optical fiber system is automatically shut off or at least reduced in case of an unintended escape of light from at least one fiber, wherein a safety interface is provided in the transmission line, said interface detecting changes of the total optical output power in at least one fiber to be received by said receiver terminal and initiating a shut-down of said input power as soon as a decrease of said total output power exceeding a predefined value is detected within a predefined time window as well as to an safety interface used to carry out such safety procedure, transporting optical signals between a transmitter terminal and a receiver terminal, with at least one pump source to enhance said optical signal, wherein said safety interface to be provided in the transmission line is capable of detecting changes of the total optical output power per fiber to be received by said receiver terminal and generating an initiating signal for a shut-down of the optical input power fed into said optical fiber system as soon as it detects a decrease of said total output power exceeding a predefined value within a predefined time window.
Especially in telecommunication fiber systems are used to transport optical signals over long distances. Such systems usually consist of reinforced cables carrying a number of individual fibers. Over long distances an important degradation of the signal to be transported can be observed. Several different approaches have successfully been undertaken to solve this problem leading to very different mechanisms. All these solutions have in common, that high optical powers are transported by the fiber, which exceed by far the hazardous threshold of the human eye. Additionally the radiation normally used in optical telecommunication belongs to the infrared portion of the electromagnetic spectrum. These long wavelengths are invisible for the human eye. This makes the radiation even more dangerous, because, when the eye is hit by such radiation, the eye closing reflex, which is based on the optical reception, cannot work properly. On the other hand the infrared radiation is very well absorbed by the aequous content of the eye, which can, hence, easily be destroyed.
Usually the radiation transported remains within the fiber system. The radiation loss leaking out of the fibers by evanescent waves is rather small and remains within the cable's optical insulation. Also the connectors between different parts of the cable or the cable and functional stations like transmitters, receivers, repeaters etc. usually are optically safe. However, an unintended escape of light from the fiber system is still possible e.g. in the case of a cable break, an equipment failure or an unintended connector unplugging. In order to avoid eye damages of persons closed to such a failure location it is necessary to provide for some safety mechanisms, which lead to an automatic shut-down of the optical power fed into the fiber. This power shall be called “total input power” in the following. The total input power consists of the signal power and any other optical power, that is fed into the system, like, for example, the pump power from pump sources in amplifier or repeater stations. A shut-down of the total input power is to be understood as complete shut-off or at least a serious reduction of the total power input, so that the light escaping from the failure location decreases rapidly under the hazardous threshold of the human eye.
The “ITU-T Recommendation G.664 (June 1999)” of the International Telecommunication Union (ITU) concerning “Optical safety procedures and requirements for optical transport systems” suggests several safety mechanisms for automatic saftety shut-down. They are all based on systems of antiparallel fiber pairs with several repeater or amplifier stations between their two main terminals (called “east” and “west” terminal). Each station contains a receiver element receiving the output power of the preceeding fiber section and a transmitter element feeding the improved signal into the succsessive fiber section. (The term “total output power” will be used in the following for the total power at the output of a fiber, i.e. the input of a receiver, be it an receiver element of an intermediate station, be it the main receiver at the main terminal. The signals of the two fibers of one pair travel in opposite directions. If there are N stations between the east terminal and the west terminal, the nth station of the east/west fiber (“station (n)1”) is located in the same housing as the (N-n+1)th station of the antiparallelly paired west/east fiber (“station (N-n+1)2”). If for example a fiber break occurs in the section between the station (n)1 and (n+1)1, viewed along the east/west direction, the total optical output power of the affected section, i.e. the optical power received by the receiver (n+1)1 drops down to almost zero. It is easy to provide the receiver elements with measuring means to detect a drop of power under a predefined threshold. In this case the transmitter (N-n)2 of the second fiber (west/east), which is located in the same housing as the receiver (n+1)1 of the first fiber (east/west), is automatically shut down. This causes a drop to almost zero of the energy received by station (N-n+1)2, which is the nearest station viewed along the west/east direction. The receiver element (N-n+1)2 is located in the same housing as the transmitter element (n)1, which feeds energy into the broken fiber section. The station (n)1/(N-n+1)2 is provided with the same automatic shut-down mechanism as the station (n+1)1/(N-n)2 mentioned before. Hence, the transmitter (n), is shut down and no hazardous light can escape from the broken fiber any more.
One drawback of this state of the art is its obvious restriction to antiparallel fiber pairs. Furthermore the safety procedure described can only be used in systems with a number of regenerator station between the east and the west terminal. In submarine systems, however, it is an important object to avoid such regenerator station for obvious reasons of maintainance. Such systems are rather provided with extremely strong pump sources near the main receiver terminal. These pump sources fed high amounts of light into the fibers in order to amplify the signal by means of Raman amplification or laser amplification in amplifier sections of the fiber, e.g. erbium-doped fiber sections. The pump light transported in the fiber together with the signal creates a large noise. As a consequence the total output power of the fiber received by the receiver terminal does not drop to a value closed to zero in case of a fiber failure. Rather a high value of noise is still measured. It is, thus, difficult to detect a cable break by a simple measurement of the total optical output power as it can be done in the state of the art cited before. Another serious problem is caused by the natural fluctuations of the pump energy. These fluctuations cause fluctuations of the absolute amount of noise as well as in the amplification gain of the signal. The resulting fluctuation of the total optical output power can by far exceed the changes of the total optical output power, which would result from a loss of signal, i.e. from a fiber break for example. The following table shows some typical values of the total optical output power at the receiver input and the pure signal power at the receiver input as a function of the pump power fed into the system.
total optical outputsignal power atpump powerpower at receiver inputreceiver input[mW](dBm)(dBm)800−8.0−10.31000−2.0−4.01100+1.5−0.2
In typical systems the pump power can easily fluctuate between the values shown in col. 1 of the table, which corresponds to a fluctuation of about 1.4 dB. As can be seen from col. 2 of the table, these pump power fluctuations cause fluctuation in the total optical output power of about 9.5 dB. From col. 3 of the table it can be read, that the signal power, which also fluctuates by about 9.5 dB, only makes a little more than half the total output power, namely about 2 dB less than the total output power. This means, that from a decrease of the total optical output power of 2 dB detected by the receiver, it cannot be decided, if a cable break, leading to a loss of signal, has occured or if a natural fluctuation of the pump power has been observed. The safety procedures according to the state of the art cited before can, thus, not be applied to unrepeatered fiber systems as they are desirable to use in submarine telecommunication systems.
On the other hand it can be seen from the table, that the pump power fed into the fiber exceeds the total output power by several orders of magnitude. Apart from the noise portion, this power travels in the opposite direction of the optical signal. However, in case of a fiber break this very high and hazardous power escapes from the broken fiber. Thus, in unrepeatered systems a reliable safety mechanism is even more crucial, than in systems with regeneration station every tens of kilometers, where no such high pump powers travel throuh the fiber.
As a partial remedy to this problem a loop back input to the pump station is used: some tens of kilometers away from the pump station a portion of the pump light traveling away from the receiver terminal, is coupled out and fed back into a receiver element located in the pump station. If a fiber failure occurs within the looped section, the pump power fed back to the pump station will dramatically drop and initiate a shut-down of the pump source. The drawbacks of such systems are evident. Only failures within the looped section are covered by this safety mechanism. On the other hand the loop cannot be extended over the whole lenght of the fiber line, because almost no feed back signal would be left after such a long travel. For this reasons usualy only the land section of the fiber line, i.e. the section between the beach line and the land based receiver terminal, is covered by the safety loop. Also within the loop the number of fibers in the cable has to be doubled. This is, together with the optical couplers necessary for coupling out the feed back portion, economically rather unfavourable.